1. Field of the Invention
The present invention relates to a liquid ejection head, more particularly to a liquid ejection head using a piezoelectric element as a pressure generating device for ejecting liquid, in order that liquid is ejected stably.
2. Description of the Related Art
As an image forming apparatus, an inkjet printer (inkjet recording apparatus) is known, which comprises an inkjet head (liquid ejection head) having an arrangement of a plurality of nozzles (ejection ports) for ejecting ink (liquid) and which forms images on a recording medium by ejecting ink from the nozzles toward the recording medium, while causing the inkjet head and the recording medium to move relatively to each other.
For example, as an ink ejection method for a inkjet recording apparatus of this kind, a piezoelectric method is known, in which a piezoelectric element is used as a pressure generating device for ejecting ink and a diaphragm which constitutes one face of a pressure chamber is deformed by the deformation of the piezoelectric element, thereby changing the volume of the pressure chamber. Consequently, ink is introduced into the pressure chamber from an ink supply passage when the volume of the pressure chamber is increased, and the ink inside the pressure chamber is ejected from a nozzle in the form of an ink droplet when the volume of the pressure chamber is decreased.
A piezoelectric element has, for example, a piezoelectric body made of lead zirconate titanate (Pb(Zr,Ti)O3 (PZT)) formed in a thin plate shape, and electrodes arranged on both surfaces of the piezoelectric body. The piezoelectric body is deformed when a voltage is applied between the electrodes. It is known that the characteristics of a piezoelectric body of this kind change with temperature. On the other hand, the viscosity of the ejected ink also changes greatly with temperature.
If a piezoelectric element is continuously driven in order to continuously eject ink when an image is formed, then the piezoelectric element is gradually heated. Therefore, when an image is formed by means of an inkjet recording apparatus, the temperature state of the inkjet head changes continually and hence the characteristics of the piezoelectric element and the viscosity of the ink change continually. Consequently, there has been a possibility that it is difficult to eject a uniform volume of ink stably, at all times. Moreover, in the case of apparatuses other than an inkjet recording apparatus, for example, a pressure sensor based on a piezoelectric element, since the characteristics of the piezoelectric element change with temperature, it is difficult to achieve uniform measurement and uniform control independently of the temperature.
In view of this, various proposals have been made for apparatuses using piezoelectric elements in order to achieve stable measurement and control, regardless of the temperature.
For example, Japanese Patent Application Publication No. 8-184520 discloses a pressure sensor which comprises a piezoelectric body that outputs determination signals in accordance with displacement of a pressure receiving rod provided inside a casing body. In the pressure sensor, a relationship whereby the thermal expansivity of the pressure receiving rod declines with respect to the thermal expansivity of the casing body is established on the basis of the temperature characteristics of the piezoelectric body. In this way, change in the piezoelectric constant is cancelled out and determination signals which are independent of the temperature are stably output.
Furthermore, for example, Japanese Patent Application Publication No. 2000-203015 discloses an apparatus which comprises an identification device for identifying the characteristics corresponding to a piezoelectric constant of a piezoelectric element of a print head, and a temperature determination device which determines the ambient temperature of the print head. In the apparatus, the drive voltage of the print head is determined and controlled on the basis of the piezoelectric constant characteristics obtained by the identification device and the temperature determined by the temperature determination device, in such a manner that the ink ejection volume is kept uniform.
However, the method described in Japanese Patent Application Publication No. 8-184520 is a method of compensating for the temperature of the piezoelectric body in a pressure sensor, and it performs the compensation by using coefficients of thermal expansion of the casing and the rod as parameters. Therefore, it is difficult to apply this method to a liquid ejection head.
Moreover, the method described in Japanese Patent Application Publication No. 2000-203015 determines the temperature of the apparatus and compensates the voltage applied to the piezoelectric element. Therefore, it requires a temperature determination system, the load on the driver is increased, the device has redundancy, and the costs are high. Furthermore, in particular, in the case of a multi-nozzle line type inkjet head, the head itself is large in size. Consequently, when this method is used in an inkjet head, temperature variation can occur in the head according to positions, and therefore it is difficult to perform the compensation completely.